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Cite this: DOI: 10.1039/c3ra46118a

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This journal is © The Royal Society of Chemistry [year] RSC Advances, 2014, 4, 1996 | 1

A novel ionic liquid for Li ion batteries – uniting the advantages of

guanidinium and piperidinium cations

Nicolas Bucher,#, a, b

Steffen Hartung,#, a, b

Denis Yu,a, d

Maria Arkhipova,

#, e Philipp Kratzer,

e Gerhard Maas,

e Madhavi Srinivasan,

a, c Harry E. Hoster

*

, a, b, d

Received (in XXX, XXX) Xth XXXXXXXXX 20XX, Accepted Xth XXXXXXXXX 20XX 5

DOI: 10.1039/c3ra46118a

We report on the synthesis and the properties of N,N,N’,N’-tetramethyl-N”,N”-pentamethyleneguanidinium bis(trifluoromethylsulfonyl)

imide (PipGuan-TFSI). The cation of this novel ionic liquid combines guanidinium and piperidinium structural elements. We tested it for

its viscosity, (Li-)ion conductivity, and also for its thermal and electrochemical stability. Furthermore, a 0.5 M solution of lithium TFSI in

PipGuan-TFSI was tested as an electrolyte for Li-ion batteries. These experiments included cycles of Li deposition/dissolution on 10

stainless steel and (de-) intercalation into/from LiFePO4 electrodes. The tests involving LiFePO4 cathodes were performed at various C-

rates and temperatures for a better quantitative comparison to other electrolyte systems. We discuss in how far PipGuan-TFSI

successfully combines the advantages of guanidinium and piperidinium ionic liquids for battery electrolyte applications.

Introduction

Ionic liquids (ILs) are salts with a melting point below 100 °C. 15

Room temperature ionic liquids, which is a subgroup defined by a

melting point below room temperature, have attracted great

interest in recent years as tunable “designer solvents”. ILs are

electrochemically and thermally stable, have a negligible vola-

tility, and thus a low flammability.1,2 Those properties make them 20

suitable not only for (sustainable) chemical synthesis3 or carbon

capture4,5 but also for energy storage applications6 such as super

capacitors, batteries, fuel cells and solar cells.2,7–12 The gain in

safety due to the low volatility of ILs comes with a higher visco-

sity and concomitantly lower ion conductivity13 as compared to 25

traditional, molecular solvents. Furthermore, any IL to be used in

electrochemical energy storage devices must exhibit a suitable

stability window. Both the ion conductivity and the potential

window are tackled by the design of new ionic liquids.14–16

IL based electrolytes in Li ion batteries (LIBs) are usually ternary 30

mixtures containing Li+ ions and the anions and cations of the

respective IL. The Li salt and the IL thus share the same anion.

Therefore, the IL cation becomes the subject of target oriented

optimization. Common basic structures are pyridinium,17 imida-

zolium,18–21 ammonium,20–23 sulfonium, pyrrolidinium,20,21,24–26 35

guanidinium,27–33or piperidinium.19–21,34,35 The infinite number of

possible structural variations in combination with the substantial

synthesis efforts calls for a systematic strategy. Parallel to

computational studies that are on the way in many research

groups,14–16 there is an obvious demand for systematic synthesis 40

variations in combination with a set of electrochemical bench-

mark tests. For substances whose properties are already close to

the desired optimum, variations can be incremental, e.g., modi-

fying moieties of existing cation structures. For substances with

more room for improvement, however, more substantial yet still 45

systematic variations may speed up the progress – in analogy to

Nature’s principles of genetic crossing that was successfully

transferred to the field of multi-parameter optimization problems.

In this paper, we will demonstrate how the advantages of two dif-

ferent structural elements can be combined towards a new cation 50

with better key properties than the parent structures. The two

original structural elements are guanidinium and piperidinium.

Guanidinium cations can be modified at six different sides and

the resulting ILs have low viscosities. Main drawback of guani-

dinium cations, however, is their lack of electrochemical stability 55

at the negative potential limit.23,27,29,30 Piperidinium based ILs, on

the other hand, were reported to have electrochemical windows

exceeding 5 V and high cathodic stability versus lithium metal.

Moreover, piperidinium ILs have a high thermal stability, which

is crucial with regard to safety.34,36 In the first part of this paper 60

we will describe the synthesis of a new cation in which one side

arm of a guanidinium cation was substituted with a piperidinium

ring (PipGuan, see Fig. 1). Being initially synthesised as PipGuan

chloride, an anion exchange towards PipGuan

bis(trifluoromethylsulfonyl)imide (TFSI) yields a new IL, 65

PipGuan-TFSI.

Fig. 1 N,N,N’,N’-Tetramethyl-N”,N”-pentamethyleneguanidinium bis(trifluoromethylsulfonyl)imide (PipGuan-TFSI). 70

2 | RSC Advances, 2014, 4, 1996 This journal is © The Royal Society of Chemistry [year]

Fig. 2 Synthesis route to PipGuan-TFSI

We will then elucidate the physical and electrochemical

properties of this IL. Those characterisations include Nuclear 5

Magnetic Resonance (NMR) and Infrared (IR) spectrometry,

Thermogravimetric analysis supported by mass spectroscopy

(TGA-MS), Differential Scanning Calorimetry (DSC), and

viscosity and conductivity measurements. The electrochemical

properties were tested by cyclic voltammetry (CV) and 10

Electrochemical Impedance Spectroscopy (EIS). Finally, we

tested a solution of LiTFSI in PipGuan-TFSI as an electrolyte for

a LiFePO4 battery half-cell and compared the results at two

different temperatures to the behaviour of the same cell using a

commercial electrolyte. We will discuss in which respect the 15

properties of the new cation structure reflect a compromise

between the two parent structures and where the product of the

crossing is superior to either of them.

Materials and Methods

Synthesis 20

The novel guanidinium-based ionic liquid N,N,N’,N’-tetramethyl-

N”,N”-pentamethyleneguanidinium bis(trifluoromethylsulfonyl)

imide was synthesised from tetramethylurea by partially

modifying reported procedures (Fig. 2).28,31 Rigorously dried

organic solvents were used. All amines were dried with KOH 25

pellets and stored under an argon atmosphere. 1H NMR spectra

were referenced to the residual proton signal of the solvent

[δ(CDCl3) = 7.26 ppm]. 13C spectra were referenced to the

solvent signal [δ(CDCl3) = 77.00 ppm], and 19F spectra to

external C6F6 [δ(C6F6) = -162.9 ppm]. 30

As a first step, N,N,N’,N’-tetramethylchloroformamidinium

chloride was obtained by a dropwise addition of tetramethylurea

(20 mL, 167 mmol) to freshly distilled oxalyl chloride (15.7 mL,

183 mmol) dissolved in 20 mL of dry dichloromethane at room

temperature. The solution was stirred overnight and the solvent 35

was removed under vacuum. The remaining precipitate was

washed several times with dry diethyl ether until the washing

ether was colourless. The solid N,N,N’,N’-tetramethylchloroform-

amidinium chloride was dried for three hours at 20 oC/0.05 mbar

to yield 27.09 g (95%) as a white moisture sensitive powder. 40

N,N,N’,N’-Tetramethyl-N”,N”-pentamethyleneguanidinium

chloride was prepared by dropwise addition of a solution of

piperidine (2.5 mL, 25 mmol) and triethylamine (3.5 mL, 25

mmol) in 20 mL of dry diethyl ether to a solution of N,N,N’,N’-

tetramethylchloroformamidinium chloride (4.3 g, 25 mmol) in 40 45

mL of dry acetonitrile at 0 °C. The mixture was stirred overnight

at room temperature. The precipitated triethylammonium chloride

was filtered off, and the solvents were distilled on the rotary

evaporator. A 0.1 M NaOH solution was added to the residual oil,

until the pH was slightly alkaline. To remove the coloured 50

impurities, the aqueous solution was washed several times with

diethyl ether. Volatile components were distilled on the rotary

evaporator (40 °C/70 mbar) and the solid residue was

subsequently dried at 50 °C/0.05 mbar. Afterwards, it was

dissolved in dry acetonitrile/diethyl ether (2 : 1 v/v, 10 mL) and 55

the solid was filtered off. The organic solvents were removed and

the product was dried for 8 h at 80 °C/0.05 mbar. Crystallisation

from ethyl acetate / dimethyl formamide (2 : 1) gave 4.2 g of

N,N,N’,N’-tetramethyl-N”,N”-pentamethyleneguanidinium

chloride (77% yield) as a white hygroscopic powder. 60


bis(trifluoromethylsulfonyl)imide (PipGuan-TFSI) was synthe-

sised by anion exchange. Lithium bis(trifluoromethylsulfonyl)-

imide (2.9 g, 10 mmol) and N,N,N’,N’-tetramethyl-N”,N”-

pentamethyleneguanidinium chloride (2.2 g, 10 mmol) were 65

dissolved in 30 and 10 mL, respectively, of deionised water. The

solutions were combined resulting in two phases. After stirring

the mixture at 70 °C for 30 min, it was cooled to room

temperature, and dichloromethane (30 mL) was added. The

organic phase was separated and washed with several portions of 70

deionised water until chloride could not be detected any more in

the rinsing water using AgNO3. The organic phase was dried with

Na2SO4, stirred over charcoal for 15 min, and filtered. The

solvent was removed on a rotary evaporator, and the product was

dried for 8 h at 120 °C/0.05 mbar. N,N,N’,N’-Tetramethyl-75

N”,N”-pentamethyleneguanidinium bis(trifluoromethylsulfonyl)-

imide was obtained as a slightly yellowish oil (4.4 g, 95% yield).

Physical and Electrochemical Characterisation

NMR spectra were recorded on a Bruker DRX 400 spectrometer

(1H: 400.13 MHz; 13C: 100.61 MHz; 19F: 376.46 MHz). IR 80

spectra were recorded with a Bruker Vector 22 FTIR

spectrometer. Thermal stability was determined by TGA (Mettler

Toledo STARe TGA/DSC1; Mettler-Toledo TGA/SDTA 851 for

N,N,N’,N’-tetramethyl-N”,N”-pentamethyleneguanidinium

chloride), the decomposition point was defined as Tdec ≙ 85

temperature of highest decomposition gradient at a heating rate of

5 K min-1. The decomposition products were detected by mass

spectrometry (Pfeiffer Vacuum Thermostar GSD320).

Differential scanning calorimetry was performed with a Perkin

Elmer DSC 7 instrument. Microanalyses were obtained with an 90

Elementar vario MICRO cube instrument. Lithium bis(trifluoro-

methylsulfonyl)imide for synthesis was received from Acros

Organics or IoLiTec GmbH with a purity of 99 %. Viscosity was

measured using a Rheometer (Anton Paar GmbH, Rheometer


MCR 501), conductivity was measured using a conductivity

meter (Eutech Instruments, CyberScan 600 Series). The

electrochemical window was measured by linear sweep

voltammetry (scan rate: 10 mV s-1) in two different cells: (i) a

two-electrode 2016 coin cell with stainless steel as the working 5

electrode and lithium as the counter and reference electrode and

(ii) a three-electrode beaker cell with glassy carbon as the

working, Ag|AgNO3 as the reference, and platinum as the counter

electrode. For the reference electrode, a Ag wire was immersed in

a solution of 0.01 M AgNO3 in acetonitrile, with 0.1 M 10

N(octyl)4BF4 as conducting salt. A junction with the

compartment containing the IL was realized with a vycor glass

frit. For both set-ups, the electrochemical windows were

calculated for the cut-off current densities 0.1 mA cm-2 and 0.5

mA cm-2. EIS was performed in a symmetric lithium coin cell set-15

up (Li|Electrolyte|Li). Plating/stripping experiments were

conducted using a 2016 coin cell set-up with lithium metal and

stainless steel electrodes. For both measurements, a Biologic

VMP3 potentiostat was used. Battery tests were performed with

an electrolyte consisting of the synthesised PipGuan-TFSI and 20

0.5 M lithium bis(trifluormethylsulfonyl)imide (LiTFSI,

SOLVAY, 99.99%). Note that the maximum concentration

attainable at 300 K was 0.8 M. Commercial lithium iron

phosphate (LFP, ENAX) was used as cathode material. The

composite cathodes were prepared by mixing LFP, acetylene 25

black (Alfa Aesar, > 99%), and polyvinylidene fluoride (PVdF,

Arkema, Kynar HSV 900) binder in the weight ratio of 80:10:10,

with N-methyl-2-pyrrolidone (NMP, Sigma Aldrich, ≥ 99%), to

form a slurry. The well-mixed, homogenous mixture was coated

on an Al foil using a doctor blade, and dried at 80 °C in air to 30

remove the solvent. Circular pieces with 16 mm diameter were

punched out of the coated Al foil and roll-pressed between twin

rollers to improve adherence of the coating to the Al foil. After

drying the electrodes for 4 h under vacuum at 110 °C, the

electrodes were assembled in a half-cell configuration in 2016 35

coin cells, using 16 mm circular lithium metal pieces as the

anode, separated by a glass fibre separator (Whatman) swollen

with the aforementioned electrolyte. Charge/discharge

experiments were carried out with an Arbin battery tester. All

measurements were performed with the purified and dry ionic 40

liquid. Magnetic resonance techniques (1H-NMR, 13C-NMR,

19F-NMR), infrared spectroscopy, mass spectroscopy, Karl-

Fischer analysis and elemental analysis did not indicate the

presence of any impurities other than small traces of water

(according to Karl-Fischer: < 50 ppm). 45

Results and Discussion

Analytical Data

N,N,N’,N’- Tetramethylchloroformamidinium chloride: 1H NMR (CDCl3): δ = 3.55 (s, 12 H, NCH3) ppm.

50


chloride

Melting point = 158–159 °C, Tdec = 330 °C, 1H NMR (CDCl3): δ

= 1.25–1.50 (m, 6 H, CH2(CH2)3CH2, pip), 2.77 (s, 12 H, NCH3),

2.97–3.13 (m, 4 H, NCH2, pip) ppm. 13C NMR (CDCl3): δ = 22.6 55

(N(CH2)2CH2, pip), 24.5 (NCH2CH2CH2, pip), 40.05 und 40.09

(NCH3), 49.3 (NCH2CH2CH2, pip), 161.9 (CN3) ppm. IR (ATR):

ν = 2930 (m), 2856 (m), 1564 (s), 1435 (m), 1407 (s), 1277 (m),

1253 (m) cm-1. MS (CI): m/z = 184 (100%, [cation]+). Anal.

calcd. for C10H22ClN3*0.75H2O: C 51.49, H 10.15, N 18.01; 60

found: C 51.48, H 10.27, N 17.94.


bis(trifluoromethylsulfonyl)imide (PipGuan-TFSI)

Melting point = 3 °C, Tdec = 463 °C, 1H NMR (CDCl3): δ = 1.65-65

1.80 (m, 6 H, CH2(CH2)3CH2, pip), 2.98 and 2.99 (2 s, 6 H each,

NCH3), 3.20–3.35 (m, 4 H, NCH2, pip) ppm. 13C NMR (CDCl3):

δ = 23.4 (N(CH2)2CH2, pip), 25.1 (NCH2CH2CH2, pip), 40.32

und 40.35 (NCH3), 49.9 (NCH2CH2CH2, pip), 162.8 (CN3) ppm.

19F NMR (CDCl3): δ = -75.3 ppm. IR (NaCl): ν = 2951 (m), 2864 70

(m), 1569 (s), 1411 (m), 1347 (s), 1330 (s), 1176 (s), 1134 (s),

1053 (s) cm-1. MS (CI): m/z = 184 (100%, [cation]+). Anal calcd.

for C12H22F6N4O4S2 (464.44): C 31.03, H 4.77, N 12.06; found: C

31.03, H 4.68, N 12.25.

Thermal properties of PipGuan-TFSI 75

DSC revealed a melting point is 3 °C as measured by DSC. It is

lower than the one of N,N-diethyl-N',N',N'',N''-tetramethyl - or

N,N,N',N'-tetramethyl N”,N”-dipropylguanidinium-TFSI (9.9 and

12.9 °C, respectively) but slightly higher than the one of

N,N,N',N'-tetramethyl-N”-ethyl-N”-propylguanidinium-TFSI (-1.6 80

°C).29,30 In general, the melting point of ionic liquids increases for

more symmetrical cations, because symmetry facilitates a closer

packing of the ions. Leaving the methyl groups at N and N’

constant, the melting point tends to decrease with increasing

chain length of the alkyl chain at N”, up to a critical molar 85

mass.30 Considering that the piperidinium contains five carbon

atoms, the relative position of the melting point of PipGuan-TFSI

is within the expected range.

TGA showed that PipGuan-TFSI is stable up to 415 °C (5% of

mass loss). The temperature of highest decomposition gradient is 90

at 463 °C. The mass spectra of the decomposition products were

analysed at the onset, in the middle, and at the end of the

decomposition (Fig. 3). In the beginning, the fragments seem to

indicate the decomposition mainly of the cation (Fig. 3a),

whereas at the end, the fragments are predominantly 95

decomposition products of the anion (Fig. 3b). In the middle of

decomposition, fragments from both cation and anion are present

(Fig. 3c). Thus, it can be assumed that the cation is the limiting

ion for thermal stability. Comparing this data with other ILs,

PipGuan-TFSI is thermally more stable than other guanidinium 100

ILs, but less stable than piperidinium based ILs.30,34


Fig. 3 a) TGA of PipGuan-TFSI (5 K min-1), b) MS of PipGuan-TFSI at 410 °C, c) MS of PipGuan-TFSI at 450 °C, d) MS of PipGuan-TFSI at 470 °C

Viscosity and Conductivity

Viscosities and conductivities were measured for pure PipGuan-

TFSI and for a solution of 0.5 M LiTFSI, a potential LIB 5

electrolyte. As found in similar systems,9,27 the viscosity of the

Li+ containing solution is higher than that of the pure IL for all

temperatures. For both samples, the viscosity decreases with

increasing temperature (Fig. 4) according to a Vogel-Tammann-

Fulcher relationship (Eq. 1);37,38 for the fitted parameters see 10

Table 1.

The values are comparable to those reported for hexaalkyl

guanidinium-based ionic liquids with TFSI as the anion.30 15

Fig. 4 Temperature dependence of viscosity for PipGuan-TFSI and 0.5 M solution of LiTFSI in PipGuan-TFSI 20

Room temperature viscosities of previously reported hexaalkyl

guanidinium30 and piperidinium34 based ILs are in the range of

58–113 mPas and 190–370 mPas, respectively. For PipGuan-

TFSI, our measurements reveal 108 mPa s at 25 °C. As expected,

this value lies between those for guanidinium and piperidinium. 25

The viscosity of the 0.5 M LiTFSI electrolyte was found to be 164

mPas at room temperature.

Conductivities were measured in an Ar filled glovebox at 28 °C.

The measurements revealed 1.46 mS cm-1 and 0.74 mS cm-1 for

pure PipGuan TFSI and the 0.5 M LiTFSI electrolyte, 30

respectively. Again, the conductivity of the pure IL lies between

the value ranges reported for N,N,N',N'-tetramethyl-N”-alkyl1-

N”-alkyl2guanidinium-TFSI (alkyl1/2 = methyl, ethyl, propyl and

butyl), which are 1.02–2.86 mS cm-1 (at 25 °C),30 and for N-

alkyl1-N-alkyl2piperidinium-TFSI (alkyl1/2 = methyl, ethyl, 35

propyl, butyl, pentyl, hexyl, or heptyl), which are 2.110-5–0.92

mS cm-1 (at 20 °C)34, respectively.

Table 1 Parameters for the Vogel-Taman-Fulcher equation for PipGuan-40

TFSI and the electrolyte (0.5 M solution of LiTFSI in PipGuan-TFSI)

0

50

100

150

200

290 300 310 320 330 340 350 360 370

Vis

cosi

ty /

mP

as

T / K

Series1

IL+0.5MLiTFSI

Plain PipGuan-TFSI

PipGuan-TFSI + 0.5M LiTFSI

(

)


Fig. 5 a) Electrochemical window of PipGuan-TFSI using a 3-electrode set-up with Ag|AgNO3 as reference, glassy carbon as working and Pt as counter electrode, b) Electrochemical window of PipGuan-TFSI using a coin cell set-up with Li as combined counter&reference and stainless 5

steel as a working electrode; the dotted lines represent a cut-off current density of 0.5 mA cm-2 for a), and 0.1 mA cm-2 for b).

Electrochemical measurements

Figs. 5a and 5b show the electrochemical window of PipGuan-

TFSI measured in two different ways. Fig. 5a was measured with 10

a three-electrode set-up, with glassy-carbon as the working,

Ag|AgNO3 as the reference, and platinum as the counter

electrode. The three-electrode set-up shows an electrochemical

window of 4.40 V between -2.55 V and 1.85 V vs. Ag|Ag+, all for

a cut-off current density of 0.5 mA cm-2. For a cut-off at 0.1 mA 15

cm-2, the window narrows to 4.16 V with -2.50 V and 1.66 V vs.

Ag|Ag+ as cathodic and anodic limits, respectively (see Table 2).

Fig. 5b was measured in a coin cell to simulate working

conditions in a LIB. The two-electrode set-up with stainless steel

as a working and Li metal as a counter electrode showed an 20

electrochemical window of 5.11 V (0.2 V … 5.31 V) for a cut-off

current density of 0.5 mA cm-2, and of 4.44 V (0.44 V… 4.88 V)

for a cut-off at 0.1 mA cm-2.

Table 2 Electrochemical stability limits and electrochemical window for

PipGuan-TFSI 25

Fig. 6 Cyclic voltammogram for 0.5 M LiTFSI in PipGuan-TFSI in coin cell set-up with stainless steel as a working and Li as a combined 30

counter&reference electrode, respectively. Scan rate 0.3 mV s-1.

It should be pointed out that the Li reference potential obtained in

these cells is not well-defined because there is no controlled Li+

content in the Li-TFSI solution as required for a true Li/Li+

system.39 Nevertheless, the upper potential limit indicates that 35

PipGuan-TFSI could be suitable for high-voltage battery cells.

For an application of PipGuan-TFSI in Li-ion batteries the elec-

trochemical properties of the Li+ containing electrolyte (PipGuan-

TFSI + 0.5 M LiTFSI) are more important than those of the plain

IL. Fig. 6 shows the first and second cycle of a freshly mounted 40

coin cell with stainless steel and lithium as working and com-

bined counter&reference electrode, respectively. Starting at 2 V,

the first CV cycle shows broad cathodic features starting at 1.5 V

in the negative going scan. Those presumably reflect electrolyte

decomposition, in analogy to previous observations for other 45

ILs.27,24,40,41

The fact that these features are only observed in the first negative

going scan indicates that the product is a passivating layer

(sometimes referred to as solid electrolyte interphase, SEI). In

following cycles, no major features are visible until -0.08 V. 50

Integration of oxidation and reduction charges shows that for the

scan from -0.08 V to -0.25 V and back, the total cathodic charge

adds up to about twice the total anodic charge that is re-gained

above -0.08 V. The anodic charge is assigned to the stripping of

Li layers that were plated onto the stainless steel electrode at E < 55

-0.08 V. The cathodic charge thus splits into contributions from

Li plating and electrolyte decomposition in coarsely equal shares.

Scans to lower E revealed a more pronounced increase of the

negative as compared to the positive charge in the anodic

counter-peak, indicating that electrolyte decomposition becomes 60

dominating at more negative potentials. Comparing the CV in

Fig. 6 with the linear sweep scan of the plain IL in Fig. 5b, the

onset of electrolyte decomposition seems to be shifted to more

negative potentials in the presence of Li+ ions in the solution.

65


Fig. 7 Impedance response of a symmetrical Li|0.5 M LiTFSI in PipGuan-TFSI|Li cell stored at room temperature during the first a) 12 hours, b) 10 days, c) time evolution of RLi

It was speculated previously that the addition of LiTFSI enables 5

the formation of a passivating layer that widens the potential

window.40,41 In-depth studies on the composition of this layer

have been conducted earlier by other groups.42–44

As highlighted above, however, the Li based reference potentials

in the plain and in the LiTFSI containing IL should not be 10

directly compared: if the potential determining equilibrium

reaction is Lisolid Li+ + e-, the Li+ concentration in the neat IL

is undefined and can easily be several orders of magnitude below

the one in the electrolyte with 0.5 M LiTFSI.39 According to the

Nernst equation the pseudo-reference potential in the neat IL can 15

thus be several multiples of 60 mV more negative than the Li/Li+

potential in the electrolyte. Such a shift could be the main reason

for the different onset potentials for cathodic IL decomposition

without (Fig. 5b) and with (Fig. 6) LiTFSI in the solution. IUPAC

recommends the utilization of dissolved redox couples (e.g., 20

ferrocene/ferrocenium) as internal references in such

systems.39,45–47 Using this method, we seek to clarify the actual

effect of LiTFSI on the width and potential stability window in

upcoming experiments. As to the potential scale in Fig. 6, one

should also emphasise that Li deposition and dissolution is setting 25

in at -0.08 V in the negative and positive going scan, respectively,

and not at 0 V as one should expect. This means that the Li/Li+

potential is more negative for the freshly deposited Li film than

for the Li foil used as combined counter&reference electrode. We

tentatively assign this potential difference to a chemical 30

modification of the surface of the Li foil due to (electro-)chemical

side reactions as they will be discussed in more detail below.

Apart from the open questions related to the electrochemical

equilibrium potentials, however, the Li plating/stripping

behaviour in Fig. 6 confirms transport of Li+ between the two 35

electrodes and thus the suitability of PipGuan-TFSI as electrolyte

solvent for Li-ion batteries.

Apart from the Li+ transport properties and the potential

dependent electrochemical stability of PipGuan-TFSI we also

studied its interaction with metallic Li. Specifically, we used EIS 40

to analyse the charge transfer properties of the interface between

metallic Li and PipGuan-TFSI + 0.5 M LiTFSI. Fig. 7 shows the

evolution of the impedance response of a cell that was stored at

room temperature under open circuit conditions, just interrupted

by the EIS measurements. The intercept of the semicircles at high 45

frequencies (left side) was associated with the bulk resistance.

The difference between the two intercepts with the real axis was

interpreted as the electrode-electrolyte resistance (RLi),

combining both the charge transfer and the passivating layer

resistance. Of these two components, RLi is probably dominated 50

by the resistance of the passivating layer.17,48,49As can be seen

from Fig. 7a, resistance stabilised after 12 hours. Fig. 7b shows

the long-term impedance test over 10 days, which shows only

little change in the RLi resistance. This indicates that a passivating

layer is forming at the lithium electrode during the first 12 hours, 55

and that it stays stable afterwards. Due to the open circuit

conditions, this formation must be a chemical or a corrosion type

of reaction.


Fig. 8 Charge/discharge behaviour of 0.5 M LiTFSI in PipGuan-TFSI in a coin cell with lithium iron phosphate and lithium electrodes; a) 24 °C, b) 55 °C.

To complete the picture of PipGuan-TFSI as a suitable electrolyte 5

solvent for Li-ion batteries we examined the charge/discharge

behaviour of LiFePO4 half cells at different C-rates and two

different temperatures (24 and 55 °C). Fig. 8a shows a discharge

capacity of 154 mAh g-1 for charge/discharge at 0.025 C. The

same capacity was found in a test experiment with the same 10

electrodes and commercial electrolyte (EC:DMC, 1 M LiPF6) at

0.2 C. For 0.05 C and 0.1 C, the discharge capacities attained

with the PipGuan-TFSI based electrolyte reduce to 143 mAh g-1

and 101 mAh g-1, respectively. Such a negative correlation of C-

rate and capacity is commonly observed and attributed to the high 15

viscosity of ionic liquids.9,19,50 This assignment fits to the

observed charge/discharge behaviour at 55 °C (Fig. 8b), where a

capacity of 148 mAh g-1 is reached at 0.2 C. This capacity is

almost 150 % of the value obtained at 0.1 C and 24 °C.

Furthermore, it is about the same capacity as we could reach for 20

an identical half cell with standard electrolyte (1 M LiPF6 in

EC/DMC) at the same C-rate and at 24 °C (curve not shown

here). In summary, the data in Fig. 8 indicate that the main

disadvantage of PipGuan-TFSI as an electrolyte solvent for Li-

ion batteries is a poor Li+ conductivity due to a high viscosity. 25

Increasing the temperature to 55 °C lowers the viscosity by about

a factor of four, which must be the reason for the improved half-

cell performance at that temperature.29 Furthermore, the achieved

discharge capacities for PipGuan-TFSI based electrolyte (143

mAh g-1 at 0.05 C, 101 mAh g-1 at 0.1 C at 24 °C ) lay above the 30

values of 93 mAh g-1 (0.05 C) and and 80 mAh g-1 (0.1 C) that

were reported for N-methyl-N-butylpiperidinium-TFSI19 and

N,N,N',N'-tetramethyl-N”-butyl-N”-methylguanidinium-TFSI,32

respectively. In this respect, the performance of the PipGuan-

TFSI based electrolyte does not lie between the two classes of 35

material but is better than the respective individual performances.

Conclusions

A new ionic liquid that combines the structures of piperidinium

and guanidinium based ionic liquids, PipGuan-TFSI, has been

synthesised. The physical properties, i.e., thermal stability, 40

viscosity and conductivity of this new material lie between these

two different classes of ionic liquids. PipGuan-TFSI has a large

electrochemical window and is able to reversibly plate and strip

lithium. Thus, it is promising as a new electrolyte in lithium ion

batteries. Its performance in a half cell equals that of commercial 45

electrolytes at low C-rates or at elevated temperatures. As

PipGuan-TFSI outperforms other common guanidinium and

piperidinium based ionic liquids with regard to achieved

capacities of LiFePO4 half cells, the synthesis goal has been

exceeded. The approach of target oriented “genetic crossing” of 50

structural elements in IL molecules is a promising complement to

computational screening and incremental modification of existing

ILs. We expect this approach to trigger further improvements in

the development of advanced electrolytes in the near future.

Acknowledgements 55

The authors would like to thank Mr. Jan Geder for helping with

the TGA-MS measurements, Ms. Shubha Nageswaran for fruitful

and interesting discussions regarding the work described in this

article, and Mr. Xiaohan Wu for his help in conducting the

experiments. The authors would also like to thank SOLVAY SA 60

for providing the LiTFSI-salt. Maria Arkhipova thanks the State

of Baden-Wuerttemberg for a graduate fellowship. This work was

financially supported by the Singapore National Research

Foundation under its Campus for Research Excellence and

Technological Enterprise (CREATE) programme. 65

Notes and references

a TUM CREATE, Singapore 138602, Singapore. Tel: +65 9455 3973;

E-mail: [email protected] b Technische Universität München, 85748 Garching,Germany. c School of Materials Science and Engineering, 70

Nanyang Technological University, Singapore 639798, Singapore. d Energy Research Institute @ NTU, 1 CleanTech Loop, #06-04

CleanTech One, Singapore 637141. e Institute of Organic Chemistry I, University of Ulm,

Albert-Einstein-Allee 11, 89081 Ulm, Germany. 75

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